The present invention relates to a propeller blade and to a propulsion device including an unducted upstream propeller and a counter-rotating and coaxial downstream propeller, and also to a method of eliminating interaction noise between the counter-rotating first and second propellers.
In the context of the present description, the terms “upstream” and “downstream” relate to the upstream and downstream directions respectively in the flow of a propulsive fluid through the two propellers. Typically, each propeller blade has a plurality of blade sections stacked along a radial axis over a height h between a blade root and a blade tip, each blade section extending between a leading edge and a trailing edge and between a pressure side and a suction side.
Pairs of unducted counter-rotating and coaxial propellers (also known as “open rotors” or as “unducted fans” (UDFs)), and in particular pairs operating at high speed, present very great potential for reducing fuel consumption, and thus for reducing the environmental impact and utilization costs in aircraft propulsion in comparison with present-day bypass turbojets. Nevertheless, such propulsion devices having unducted counter-rotating and coaxial high-speed propellers on the same axis present the drawback of relatively high levels of sound emission.
One of the main factors contributing to these sound emissions lies in the interaction between the counter-rotating propellers, and in particular in blade tip vortices of the first propeller being carried away in the flow direction of the propulsive fluid and impacting against the blades of the second propeller located downstream. As well as the noise nuisance, the pressure gradient generated by such impacts on the surface of the second propeller can reduce its propulsive efficiency and can also constitute an additional source of mechanical fatigue for the blades of the second propeller.
A first solution that has been proposed for this problem lies in reducing the diameter of the downstream propeller so that the blade tip vortices of the upstream propeller pass outside the blade tips of the downstream propeller and do not interact therewith. Such clipping of the blade tips of the downstream propeller is not satisfactory since it reduces propulsion performance.
Another solution, proposed in U.S. Pat. No. 5,190,441, consists in significantly increasing the dihedral angle of the blade tips of the upstream propeller so as to weaken the blade tip vortices in a manner analogous to the vortex weakening provided by the winglets installed at the ends of the wings of certain aircraft in order to reduce the effects of wing tip vortices. Nevertheless, apart from presenting relatively limited effectiveness in reducing blade tip vortices, that solution also presents structural drawbacks.
In yet another solution, proposed in French patent application FR 2 935 349, a stream of air is ejected at the blade tips of the upstream propeller in order to weaken the blade tip vortices by interacting with the boundary layer. In similar manner, proposals are made in European patent application EP 2 287 072 A2 to eject such an air stream at the blade tips in order to generate a secondary vortex for the purpose of destabilizing the blade tip vortex. Friction between two parallel corotating vortices is capable of destabilizing and indeed of destroying both vortices, even when the intensities of the two vortices are very different.